1. Field of the Invention
This invention relates to an improved process for making copper oxide-containing superconductors.
2. Description of Related Art
Bednorz and Muller, Z. Phys. B64, 189-193 (1986), disclose a superconducting phase in the La-Ba-Cu-O system with a superconducting transition temperature of about 35 K. Samples were prepared by a coprecipitation method from aqueous solutions of Ba-, La- and Cu-nitrate in their appropriate ratios. An aqueous solution of oxalic acid was used as the precipitant. Chu et al., Phys. Rev. Lett. 58, 405-407 (1987), report detection of an apparent superconducting transition with an onset temperature above 40 K. under pressure in the La-Ba-Cu-O compound system synthesized directly from a solid-state reaction of La.sub.2 O.sub.3, CuO and BaCO.sub.3 followed by a decomposition of the mixture in a reduced atmosphere. Chu et al., Science 235, 567-569 (1987), disclose that a superconducting transition with an onset temperature of 52.5 K. has been observed under hydrostatic pressure in compounds with nominal compositions given by (La.sub.0.9 Ba.sub.0.1).sub.2 CuO.sub.4-y, where y is undetermined. They state that the K.sub.2 NiF.sub.4 layer structure has been proposed to be responsible for the high-temperature superconductivity in the La-Ba-Cu-O system (LBCO). They further state that, however, the small diamagnetic signal, in contrast to the presence of up to 100% K.sub.2 NiF.sub.4 phase in their samples, raises a question about the exact location of superconductivity in LBCO. Cava et al., Phys. Rev. Lett. 58, 408-410 (1987), disclose bulk superconductivity at 36 K. in La.sub.1.8 Sr.sub.0.2 CuO.sub.4 prepared from appropriate mixtures of high purity La(OH).sub.3, SrCO.sub.3 and CuO powders, heated for several days in air at 1000.degree. C. in quartz crucibles. Rao et al., Current Science 56, 47-49 (1987), discuss superconducting properties of compositions which include La.sub.1.8 Sr.sub.0.2 CuO.sub.4, La.sub.1.85 Ba.sub.0.15 CuO.sub.4, La.sub.1.8 Sr.sub.0.1 CuO.sub.4, (La.sub.1-x Pr.sub.x).sub.2-y Sr.sub.y CuO.sub.4, and (La.sub.1.75 Eu.sub.0.25)Sr.sub.0.2 CuO.sub.4. Bednorz et al., Europhys. Lett. 3, 379-384 (1987), report that susceptibility measurements support high-T.sub.c superconductivity in the Ba-La-Cu-O system. In general, in the La-Ba-Cu-O system, the superconducting phase has been identified as the composition La.sub.1-x (Ba,Sr,Ca).sub.x CuO.sub.4-y with the tetragonal K.sub.2 NiF.sub.4 -type structure and with x typically about 0.15 and y indicating oxygen vacancies.
Wu et al., Phys. Rev. Lett. 58, 908-910 (1987), disclose a superconducting phase in the Y-Ba-Cu-O system with a superconducting transition temperature between 80 and 93 K. The compounds investigated were prepared with nominal composition (Y.sub.1-x Ba.sub.x).sub.2 CuO.sub.4-y and x=0.4 by a solid-state reaction of appropriate amounts of Y.sub.2 O.sub.3, BaCO.sub.3 and CuO in a manner similar to that described in Chu et al., Phys Rev. Lett. 58, 405-407 (1987). Said reaction method comprises more specifically heating the oxides in a reduced oxygen atmosphere of 2.times.10.sup.-5 bars (2 Pa) at 900.degree. C. for 6 hours. The reacted mixture was pulverized and the heating step was repeated. The thoroughly reacted mixture was then pressed into 3/16 inch (0.5 cm) diameter cylinders for final sintering at 925.degree. C. for 24 hours in the same reduced oxygen atmosphere. The material prepared showed the existence of multiple phases.
Hor et al., Phys. Rev. Lett. 58, 911-912 (1987), disclose that pressure has only a slight effect on the superconducting transition temperature of the Y-Ba-Cu-O superconductors described by Wu et al., supra.
Sun et al., Phys. Rev. Lett. 58, 1574-1576 (1987), disclose the results of a study of Y-Ba-Cu-O samples exhibiting superconductivity with transition temperatures in the 90 K. range. The samples were prepared from mixtures of high-purity Y.sub.2 O.sub.3, BaCO.sub.3 and CuO powders. The powders were premixed in methanol or water and subsequently heated to 100.degree. C. to evaporate the solvent. Two thermal heat treatments were used. In the first, the samples were heated in Pt crucibles for 6 hours in air at 850.degree. C. and then for another 6 hours at 1000.degree. C. After the first firing, the samples were a dark-green powder, and after the second firing, they became a very porous, black solid. In the second method, the powders were heated for 8-10 hours at 1000.degree. C., ground and then cold pressed to form disks of about 1 cm diameter and 0.2 cm thickness. The superconducting properties of samples prepared in these two ways were similar. X-ray diffraction examination of the samples revealed the existence of multiple phases.
Cava et al., Phys. Rev. Lett. 58, 1676-1679 (1987), have identified this superconducting Y-Ba-Cu-O phase to be orthorhombic, distorted, oxygen-deficient perovskite YBa.sub.2 Cu.sub.3 O.sub.9-.delta. where .delta. is about 2.1, and have presented the X-ray diffraction powder pattern and lattice parameters for the phase. The single-phase YBa.sub.2 Cu.sub.3 O.sub.9-.delta. was prepared in the following manner. BaCO.sub.3, Y.sub.2 O.sub.3 and CuO were mixed, ground and then heated at 950.degree. C. in air for 1 day. The material was then pressed into pellets, sintered in flowing O.sub.2 for 16 hours and cooled to 200.degree. C. in O.sub.2 before removal from the furnace. Additional overnight treatment in O.sub.2 at 700.degree. C. was found to improve the observed properties.
Takita et al., Jpn. J. Appl. Phys. 26, L506-L507 (1987), disclose the preparation of several Y-Ba-Cu compositions with superconducting transitions around 90 K. by a solid-state reaction method in which a mixture of Y.sub.2 O.sub.3, CuO, and BaCO.sub.3 was heated in an oxygen atmosphere at 950.degree. C. for more than 3 hours. The reacted mixture was pressed into 10 mm diameter disks for final sintering at 950.degree. or 1000.degree. C. for about 3 hours in the same oxygen atmosphere.
Takabatake et al., Jpn. J. Appl. Phys. 26, L502-L503 (1987), disclose the preparation of samples of Ba.sub.1-x Y.sub.x CuO.sub.3-z (x=0.1, 0.2, 0.25, 0.3, 0.4, 0.5, 0.6, 0.8 and 0.9) from the appropriate mixtures of BaCO.sub.3, Y.sub.2 O.sub.3 and CuO. The mixture was pressed into a disc and sintered at 900.degree. C. for 15 hours in air. The sample with x=0.4 exhibited the sharpest superconducting transition with an onset near 96 K.
Syono et al., Jpn. J. Appl. Phys. 26, L498-L501 (1987), disclose the preparation of samples of superconducting Y.sub.0.4 Ba.sub.0.6 CuO.sub.2.22 with T.sub.c higher than 88 K. by firing mixtures of 4N Y.sub.2 O.sub.3, 3N BaCO.sub.3 and 3N CuO in the desired proportions. The mixtures were prefired at 1000.degree. C. for 5 hours. They were ground, pelletized and sintered at 900.degree. C. for 15 hours in air and cooled to room temperature in the furnace. They also disclose that almost equivalent results were also obtained by starting from concentrated nitrate solution of 4N Y.sub.2 O.sub.3, GR grade Ba(NO.sub.3).sub.2 and Cu(NO.sub.3).sub.2.
Takayama-Muromachi et al., Jpn. J. Appl. Phys. 26, L476-L478 (1987), disclose the preparation of a series of samples to try to identify the superconducting phase in the Y-Ba-Cu-O system. Appropriate amounts of Y.sub.2 O.sub.3, BaCO.sub.3 and CuO were mixed in an agate mortar and then fired at 1173.+-.2 K. for 48-72 hours with intermediate grindings. X-ray diffraction powder patterns were obtained. The suggested composition of the superconducting compound in Y.sub.1-x Ba.sub.x CuO.sub.y where 0.6&lt;.times.&lt;0.7.
Hosoya et al., Jpn. J. Appl. Phys. 26, L456-L457 (1987), disclose the preparation of various superconductor compositions in the L-Ba-Cu-O systems where L=Tm, Er, Ho, Dy, Eu and Lu. Mixtures of the proper amounts of the lanthanide oxide (99.9% pure), CuO and BaCO.sub.3 were heated in air. The obtained powder specimens were reground, pressed into pellets and heated again.
Hirabayashi et al., Jpn. J. Appl. Phys. 26, L454-L455 (1987), disclose the preparation of superconductor samples of nominal composition Y.sub.1/3 Ba.sub.2/3 CuO.sub.3-x by coprecipitation from aqueous nitrate solution. Oxalic acid was used as the precipitant and insoluble Ba, Y and Cu compounds were formed at a constant pH of 6.8. The decomposition of the precipitate and the solid-state reaction were performed by firing in air at 900.degree. C. for 2 hours. The fired products were pulverized, cold-pressed into pellets and then sintered in air at 900.degree. C. for 5 hours. The authors found that the sample was of nearly single phase having the formula Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7. The diffraction pattern was obtained and indexed as having tetragonal symmetry.
Ekino et al., Jpn. J. Appl. Phys. 26, L452-L453 (1987), disclose the preparation of a superconductor sample with nominal composition Y.sub.1.1 Ba.sub.0.9 CuO.sub.4-y. A prescribed amount of powders of Y.sub.2 O.sub.3, BaCO.sub.3 and CuO was mixed for about an hour, pressed under 6.4 ton/cm.sup.2 (14 MPa) into pellet shape and sintered at 1000.degree. C. in air for 3 hours.
Akimitsu et al., Jpn. J. Appl. Phys. 26, L449-L451 (1987), disclose the preparation of samples with nominal compositions represented by (Y.sub.1-x Ba.sub.x).sub.2 CuO.sub.4-y. The specimens were prepared by mixing the appropriate amounts of powders of Y.sub.2 O.sub.3, BaCO.sub.3 and CuO. The resulting mixture was pressed and heated in air at 1000.degree. C. for 3 hours. Some samples were annealed at appropriate temperatures in O.sub.2 or CO.sub.2 for several hours. The authors noted that there seemed to be a tendency that samples annealed in O.sub.2 showed a superconducting transition with a higher onset temperature but a broader transition than non-annealed samples.
Semba et al., Jpn. J. Appl. Phys. 26, L429-L431 (1987), disclose the preparation of samples of Y.sub.x Ba.sub.1-x CuO.sub.4-d where x=0.4 and x=0.5 by the solid state reaction of BaCO.sub.3, Y.sub.2 O.sub.3 and CuO. The mixtures are heated to 950.degree. C. for several hours, pulverized, and then pressed into disk shape. This is followed by the final heat treatment at 1100.degree. C. in one atmosphere O.sub.2 gas for 5 hours. The authors identified the phase that exhibited superconductivity above 90 K. as one that was black with the atomic ratio of Y:Ba:Cu of 1:2:3. The diffraction pattern was obtained and indexed as having tetragonal symmetry.
Hatano et al., Jpn. J. Appl. Phys. 26, L374-L376 (1987), disclose the preparation of the superconductor compound Ba.sub.0.7 Y.sub.0.3 Cu.sub.1 O.sub.x from the appropriate mixture of BaCO.sub.3 (purity 99.9%), Y.sub.2 O.sub.3 (99.99%) and CuO (99.9%). The mixture was calcined in an alumina boat heated at 1000.degree. C. for 10 hours in a flowing oxygen atmosphere. The color of the resulting well-sintered block was black.
Hikami et al., Jpn. J. Appl. Phys. 26, L347-L348 (1987), disclose the preparation of a Ho-Ba-Cu oxide, exhibiting the onset of superconductivity at 93 K. and the resistance vanishing below 76 K., by heating a mixture of powders Ho.sub.2 O.sub.3, BaCO.sub.3 and CuO with the composition Ho:Ba:Cu=0.246:0.336:1 at 850.degree. C. in air for two hours. The sample was then pressed into a rectangular shape and sintered at 800.degree. C. for one hour. The sample looked black, but a small part was green.
Matsushita et al., Jpn. J. Appl. Phys. 26, L332-L333 (1987), disclose the preparation of Ba.sub.0.5 Y.sub.0.5 Cu.sub.1 O.sub.x by mixing appropriate amounts of BaCO.sub.3 (purity 99.9%), Y.sub.2 O.sub.3 (99.99%) and CuO (99.9%). The mixture was calcined at 1000.degree. C. for 11 hours in a flowing oxygen atmosphere. The resultant mixture was then pulverized and cold-pressed into disks. The disks were sintered at 900.degree. C. for 4 hours in the same oxygen atmosphere. The calcined powder and disks were black. A superconducting onset temperature of 100 K. was observed.
Maeno et al., Jpn. J. Appl. Phys. 26, L329-L331 (1987), disclose the preparation of various Y-Ba-Cu oxides by mixing powders of Y.sub.2 O.sub.3, BaCO.sub.3 and CuO, all 99.99% pure, with a pestle and mortar. The powders were pressed at 100 kgf/cm.sup.2 (98.times.10.sup.4 Pa) for 10-15 minutes to form pellets with a diameter of 12 mm. The pellets were black. The heat treatment was performed in two steps in air. First, the pellets were heated in a horizontal, tubular furnace at 800.degree. C. for 12 hours before the heater was turned off to cool the pellets in the furnace. The pellets were taken out of the furnace at about 200.degree. C. About half the samples around the center of the furnace turned green in color, while others away from the center remained black. The strong correlation with location suggested to the authors that this reaction occurs critically at about 800.degree. C. The pellets were then heated at 1200.degree. C. for 3 hours and then allowed to cool. Pellets which turned light green during the first heat treatment became very hard solids whereas pellets which remained black in the first heat treatment slightly melted or melted down. Three of the samples exhibited an onset of superconductivity above 90 K.
Iguchi et al., Jpn. J. Appl. Phys. 26, L327-L328 (1987), disclose the preparation of superconducting Y.sub.0.8 Ba.sub.1.2 CuO.sub.y by sintering a stoichiometrical mixture of Y.sub.2 O.sub.3, BaCO.sub.3 and CuO at 900.degree. C. and at 1000.degree. C. in air.
Hosoya et al., Jpn. J. Appl. Phys. 26, L325-L326 (1987), disclose the preparation of various superconducting specimens of the L-M-Cu-O systems where L=Yb, Lu, Y, La, Ho and Dy and M=Ba and a mixture of Ba and Sr by heating the mixtures of appropriate amounts of the oxides of the rare earth elements (99.9% pure), CuO, SrCO.sub.3 and/or BaCO.sub.3 in air at about 900.degree. C. Green powder was obtained. The powder samples were pressed to form pellets which were heated in air until the color became black.
Takagi et al., Jpn. J. Appl. Phys. 26, L320-L321 (1987), disclose the preparation of various Y-Ba-Cu oxides by reacting mixtures containing the prescribed amounts of powders of Y.sub.2 O.sub.3, BaCO.sub.3 and CuO at 1000.degree. C., remixing and heat-treating at 1100.degree. C. for a few to several hours. An onset temperature of superconductivity at 95 K. or higher was observed for a specimen with the nominal composition of (Y.sub.0.9 Ba.sub.0.1)CuO.sub.y.
Hikami et al., Jpn. J. Appl. Phys. 26, L314-L315 (1987), disclose the preparation of compositions in the Y-Ba-Cu-O system by heating the powders of Y.sub.2 O.sub.3, BaCO.sub.3 and CuO to 800.degree. C. or 900.degree. C. in air for 2-4 hours, pressing into pellets at 4 kbars (4.times.10.sup.5 Pa) and reheating to 800.degree. C. in air for 2 hours for sintering. The samples show an onset of superconductivity at 85 K. and a vanishing resistance at 45 K.
Bourne et al., Phys. Letters A 120, 494-496 (1987), disclose the preparation of Y-Ba-Cu-O samples of Y.sub.2-x Ba.sub.x CuO.sub.4 by pressing finely ground powders of Y.sub.2 O.sub.3, BaCO.sub.3 and CuO into pellets and sintering the pellets in an oxygen atmosphere at 1082.degree. C. Superconductivity for samples having x equal to about 0.8 was reported.
Moodenbaugh et al., Phys. Rev. Lett. 58, 1885-1887 (1987), disclose superconductivity near 90 K. in multiphase samples with nominal composition Lu.sub.1.8 Ba.sub.0.2 CuO.sub.4 prepared from dried Lu.sub.2 O.sub.3, high-purity BaCP.sub.3 (BaCO.sub.3 presumably), and fully oxidized CuO. These powders were ground together in an agate mortar and then fired overnight in air at 1000.degree. C. in Pt crucibles. This material was ground again, pelletized, and then fired at 1100.degree. C. in air for 4-12 hours in Pt crucibles. Additional samples fired solely at 1000.degree. C. and those fired at 1200.degree. C. show no signs of superconductivity.
Hor et al., Phys. Rev. Lett. 58, 1891-1894 (1987), disclose superconductivity in the 90 K. range in ABa.sub.2 Cu.sub.3 O.sub.6+x with A=La, Nd, Sm, Eu, Gd, Ho, Er, and Lu in addition to Y. The samples were synthesized by the solid-state reaction of appropriate amounts of sesquioxides of La, Nd, Sm, Eu, Gd, Ho, Er, and Lu, BaCO.sub.3 and CuO in a manner similar to that described in Chu et al., Phys. Rev. Lett. 58, 405 (1987) and Chu et al., Science 235, 567 (1987).
Morgan, "Processing of Crystalline Ceramics", Palmoor et al., eds., Plenum Press, New York, 67-76 (1978), in discussing chemical processing for ceramics states that where direct synthesis is not immediately achieved, the use of co-precipitation, even if not completely homogeneous on a molecular scale is so vastly superior for uniform powder preparation to the use of ball milled oxides, that it should be the method of choice. He further discusses conditions for preparing perovskites or potassium nickel fluoride type structures using the oxides of Ca, Sr, Li, lanthanides, etc. and hot solutions of transition metal nitrates and acetates.
C. Michel et al., Z. Phys. B - Condensed Matter 68, 421 (1987), disclose a novel family of superconducting oxides in the Bi-Sr-Cu-O system with composition close to Bi.sub.2 Sr.sub.2 Cu.sub.2 O.sub.7+.delta.. A pure phase was isolated for the composition Bi.sub.2 Sr.sub.2 Cu.sub.2 O.sub.7+.delta.. The X-ray diffraction pattern for this material exhibits some similarity with that of perovskite and the electron diffraction pattern shows the perovskite subcell with the orthorhombic cell parameters of a=5.32 A (0.532 nm), b=26.6 A (2.66 nm) and c=48.8 A (4.88 nm). The material made from ultrapure oxides has a superconducting transition with a midpoint of 22 K. as determined from resistivity measurements and zero resistance below 14 K. The material made from commercial grade oxides has a superconducting transition with a midpoint of 7 K. In each case a mixture of Bi.sub.2 O.sub.3, CuO and SrCO.sub.3 was heated at 800.degree. C. for 12 hours in air, sintered at 900.degree. C. for 2 hours in air and quenched to room temperature.
Akimitsu et al., Jpn. J. Appl. Phys. 26, L2080 (1987) disclose a Bi-Sr-Cu-O composition with a superconducting onset temperature of near 8 K. made by firing well-dried mixtures of Bi.sub.2 O.sub.3, SrCO.sub.3 and CuO powders at about 880.degree. C. in air for 12 hours.
H. Maeda et al., Jpn. J. Appl. Phys. 27, L209 (1988), disclose a superconducting oxide in the Bi-Sr-Ca-Cu-O system with the composition near BiSrCaCu.sub.2 Ox and a superconducting transition temperature T.sub.c of about 105 K. The samples were prepared by calcining a mixture of Bi.sub.2 O.sub.3, SrCO.sub.3, CaCO.sub.3, and CuO powders at 800.degree.-870.degree. C. for 5 hours. The material was reground, cold-pressed into pellets at a pressure of 2 ton/cm.sup.2, sintered at about 870.degree. C. in air or oxygen and furnace-cooled to room temperature.
The commonly assigned application, "Superconducting Metal Oxide Compositions and Process For Making Them", Ser. No. 153,107, filed Feb. 8, 1988, a continuation-in-part of Ser. No. 152,186, filed Feb. 4, 1988, disclose superconducting compositions having the nominal formula Bi.sub.a Sr.sub.b Ca.sub.c Cu.sub.3 O.sub.x wherein a is from about 1 to about 3, b is from about 3/8 to about 4, c is from about 3/16 to about 2 and x=(1.5 a+b+c+y) where y is from about 2 to about 5, with the proviso that b+ c is from about 3/2 to about 5, said compositions having superconducting transition temperatures of about 70 K. or higher. It also discloses the superconducting metal oxide phase having the formula Bi.sub.2 Sr.sub.3-z Ca.sub.z Cu.sub.2 O.sub.8+w wherein z is from about 0.1 to about 0.9, preferably 0.4 to 0.8 and w is greater than zero but less than about 1. M. A. Subramanian et al., Science 239, 1015 (1988) also disclose the Bi.sub.2 Sr.sub.3-z Ca.sub.z Cu.sub.2 O.sub.8+w superconductor and a method for preparing it. Single crystals were grown from a mixture of Bi.sub.2 O.sub.3, CaCO.sub.3, SrO.sub.2 /Sr(NO.sub.3).sub.2 and CuO in proportions such that the atomic ratio Bi:Sr:Ca:Cu=2:2:1:3. The mixture was heated to 850.degree. to 900.degree. C., held for 36 hours and cooled at the rate of 1.degree. C. per minute.
L. F. Schneemeyer et al., Nature 332, 422 (1988), disclose an alkali chloride flux method to grow superconducting single crystals in the Bi-Sr-Ca-Cu-O system. Pre-reacted Bi-Sr-Ca-Cu-O mixtures were prepared from high-purity Bi.sub.2 O.sub.3, SrCO.sub.3, Ca(OH).sub.2, and CuO by slowly heating to 800.degree.-850.degree. C. with intermediate grinding steps. Charges consisting of 10-50 wt % Bi-Sr-Ca-Cu-O mixtures thoroughly mixed with NaCl, KCl or other alkali halide salt or salt mixtures were placed in crucibles, heated above the melting temperature if the salt or salt mixture, and cooled at rates of 1.degree.-10.degree. C. per hour.
S. Kondoh et al., Solid State Comm. 65, 1329 (1988), disclose a superconductor composition in the Tl-Ba-Cu-O system with an onset of superconductivity as high as 20 K. These superconductors were prepared by heating a mixture of Tl.sub.2 O.sub.3, BaO and CuO at about 650.degree. C. for 12 hours. The producy was reground, pressed into pellets and heated again at 700.degree. C. for about 20 hours.
Z. Z. Sheng et al., Nature 332, 55 (1988) and Z. Z. Sheng et al., Phys. Rev. Lett. 60, 937 (1988) disclose superconductivity in the Tl-Ba-Cu-O system. These samples are reported to have onset temperatures above 90 K. and zero resistance at 81 K. The samples were prepared by mixing and grinding appropriate amounts of BaCO.sub.3 and CuO with an agate mortar and pestle. This mixture was heated in air at 925.degree. C. for more than 24 hours with several intermediate grindings to obtain a uniform black oxide Ba-Cu oxide powder which was mixed with an appropriate amount of Tl.sub.2 O.sub.3, completely ground and pressed into a pellet with a diameter of 7 mm and a thickness of 1-2 mm. The pellet was then put into a tube furnace which had been heated to 880.degree.-910.degree. C. and was heated for 2-5 minutes in flowing oxygen. As soon as it had slightly melted, the sample was taken from the furnace and quenched in air to room temperature. It was noted by visual inspection that Tl.sub.2 O.sub.3 had partially volatilized as black smoke, part had become a light yellow liquid, and part had reacted with Ba-Cu oxide forming a black, partially melted, porous material.
Z. Z. Sheng et al., Nature 332, 138 (1988) disclose superconductivity in the Tl-Ba-Ca-Cu-O system. Samples are reported to have onset temperatures at 120 K. and zero resistance above 100 K. The samples were prepared by mixing and grinding appropriate amounts of Tl.sub.2 O.sub.3, CaO and BaCuO.sub.4. This mixture was pressed into a pellet with a diameter of 7 mm and a thickness of 1-2 mm. The pellet was then put into a tube furnace which had been heated to 880.degree.-910.degree. C. and was heated for 3-5 minutes in flowing oxygen. The sample was then taken from the furnace and quenched in air to room temperature.
R. M. Hazen et al., Phy. Rev. Lett. 60, 1657 (1988), disclose two superconducting phases in the Tl-Ba-Ca-Cu-O system, Tl.sub.2 Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10 and Tl.sub.2 Ba.sub.2 CaCu.sub.2 O.sub.8 and the method of preparation. Appropriate amounts of Tl.sub.2 O.sub.3, CaO, and BaCu.sub.3 O.sub.4 or Ba.sub.2 Cu.sub.3 O.sub.4 were completely mixed, ground and pressed into a pellet. A quartz boat containing the pellet was placed in a tube furnace which had been preheated to 880.degree.-910.degree. C. The sample was heated for 3 to 5 minutes in flowing oxygen and the furnace cooled to room temperature in 1 to 1.5 hours.
Subramanian et al., Nature 332, 420 (1988) disclose the preparation of superconducting phases in the Tl-Ba-Ca-Cu-O system by reacting Tl.sub.2 O.sub.3, CaO.sub.2 or CaCO.sub.3, BaO.sub.2 and CuO at 850.degree.-910.degree. C. in air or in sealed gold tubes for 15 minutes to 3 hours.